“We do not believe any group of men adequate enough or wise enough to operate without scrutiny or without criticism. We know that the only way to avoid error is to detect it, that the only way to detect it is to be free to inquire. We know that in secrecy error undetected will flourish and subvert”. – J Robert Oppenheimer.

The event was climaxed by a warmer, more intense storm with much more rain that was much more serious, due to the earlier large accumulation of snow, now melted by the large turbulent heat fluxes into the snow over the lower elevations of the mountains. Throughout the affected area, all the streams and rivers rose to great heights, flooded the valleys, inundated or swept away towns, mills, dams, flumes, houses, fences, and domestic animals, and ruined fields. An early estimate of property damage was $10,000,000. However, later it was estimated that approximately one-quarter of the taxable real estate in the state of California was destroyed in the flood. Dependent on property taxes, the State of California went bankrupt. The governor, state legislature, and state employees were not paid for a year and a half. 200,000 cattle drowned, and the state’s economy shifted from ranching to farming.

In recent years the flood has held the attention of the USGS and emergency planners, who used it as an example when modelling the impact of a similar event happening in modern-day California. The official name for such an event is "The Arkstorm", and it is unofficially called "The other big one".

Background

The floods were likely caused by precipitation from atmospheric rivers, or narrow bands of water vapor about a mile above sea level that extend for thousands of kilometers.

Prior to the flooding, Oregon had steady but heavier than normal rainfall during November and heavier snow in the mountains.

The weather pattern that caused this flood was not from an El Nino, and from the existing Army and private weather records, it has been determined that the polar jet stream was to the north as the Pacific Northwest experienced a mild rainy pattern for the first half of December 1861. The jet stream then slid south and freezing conditions were reported at Oregon stations by December 25. Heavy rainfall began falling in California as the longwave trough moved down over the state, remaining there until the end of January 1862 and causing precipitation everywhere in the state for nearly 40 days. Eventually the trough moved even further south, causing snow to fall in the Central Valley and surrounding mountain ranges.

The ARkStorm Scenario

The US Geological Society, USGS, has run a simulation, known as the ARkStorm Scenario, to forecast what damage a similar event to the Great Flood would have today. They conclude:

This document summarizes the next major public project for MHDP, Multi Hazards Demonstration Project, a winter storm scenario called ARkStorm (for Atmospheric River 1,000). Experts have designed a large, scientifically realistic meteorological event followed by an examination of the secondary hazards (for example, landslides and flooding), physical damages to the built environment, and social and economic consequences. The hypothetical storm depicted here would strike the U.S. West Coast and be similar to the intense California winter storms of 1861 and 1862 that left the central valley of California impassable. The storm is estimated to produce precipitation that in many places exceeds levels only experienced on average once every 500 to 1,000 years.

Extensive flooding results. In many cases flooding overwhelms the state’s flood-protection system, which is typically designed to resist 100- to 200-year runoffs. The Central Valley experiences hypothetical flooding 300 miles long and 20 or more miles wide. Serious flooding also occurs in Orange County, Los Angeles County, San Diego, the San Francisco Bay area, and other coastal communities. Windspeeds in some places reach 125 miles per hour, hurricane-force winds. Across wider areas of the state, winds reach 60 miles per hour. Hundreds of landslides damage roads, highways, and homes. Property damage exceeds $300 billion, most from flooding. Demand surge (an increase in labor rates and other repair costs after major natural disasters) could increase property losses by 20 percent. Agricultural losses and other costs to repair lifelines, dewater (drain) flooded islands, and repair damage from landslides, brings the total direct property loss to nearly $400 billion, of which $20 to $30 billion would be recoverable through public and commercial insurance. Power, water, sewer, and other lifelines experience damage that takes weeks or months to restore. Flooding evacuation could involve 1.5 million residents in the inland region and delta counties. Business interruption costs reach $325 billion in addition to the $400 billion property repair costs, meaning that an ARkStorm could cost on the order of $725 billion, which is nearly 3 times the loss deemed to be realistic by the ShakeOut authors for a severe southern California earthquake, an event with roughly the same annual occurrence probability.

We often hear of one in a hundred year events, but this was a once every 500 or 1000 years event. And, of course, it was not a localised event, such as the Colorado floods last year.

It is also worth noting the role of the jet stream, although nobody at the time knew of it. We know that movement of the jet stream nowadays, along with blocking systems can lead to extreme weather, but clearly this is nothing new.

The cost of such an event today is given as $400 billion for property, and $725 billion for all costs. we can contrast this with Katrina, which is reckoned to have cost about $100 billion and $250 billion respectively.

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I was in Sacramento earlier this year. One of the features of the old town near the river is that after great floods in 1850 and 1862 it was rebuilt at a higher level. Some parts of the old lower streets can still be seen. An early example of adapting to climate rather than thinking you can change it?

“In 1853 a mammoth project was proposed to raise the city above the flood level. The ambitious and expensive proposal was not fully accepted until another devastating flood swept through the city in 1862. Within a few years, thousands of cubic yards of earth were brought in on wagons and the daring scheme to raise the street level began. The original street level can be seen throughout Old Sacramento under the boardwalks and in basement”